Method for battery equalization

ABSTRACT

A battery equalization circuit is provided, including: a positive battery node connecting to a positive node of a battery cell in a battery circuit with a plurality of other battery cells; a negative battery node connected to a negative node of the battery cell; a transformer winding receiving an AC voltage, the transformer winding having an upper transformer node and a lower transformer node; an upper triac connected between the positive battery node and the upper transformer node; a lower triac connected between the negative battery node and the lower transformer node; a control circuit for controlling the upper triac and the lower triac based on a measured cell voltage between the positive battery node and the negative battery node, and a total battery voltage of the battery circuit; and an isolation element connected between the control circuit and a data bus.

This application is a divisional application of a U.S. patentapplication entitled “Circuit and Method for Battery Equalization”,having a Ser. No. 13/770,170, having a filing date of Feb. 19, 2013,having common inventors, and having a common assignee, all of which isincorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates in general to a system and method forcontrolling the equalization of battery cells in a multiple-cellbattery. In particular it relates to a system and method for monitoringthe voltage in each battery cell in a multiple-cell battery, as well asthe in the battery as a whole, and for equalizing the voltages in eachof the individual battery cells so that they are approximately the same.

BACKGROUND OF THE INVENTION

Some simple batteries are made up of a single cell having a singlevoltage. For example, a common A cell battery is made up of a single 1.5volt cell. However, many other batteries are made up of multiple batterycells that are connected together in series. For example, a common carbattery could be made up of six 2 volt cells connected in series, whilea battery for a hybrid car might be made up of an array of batterycells, some in series and some in parallel. Likewise, an electronicdevice might employ a battery made of a chain of smaller batteriesconnected in series.

In multiple-cell batteries, small differences between the cells (e.g.,due to production tolerances or operating conditions) tend to bemagnified with each charge or discharge cycle. In these situations,weaker cells are overstressed during charging, causing them to becomeeven weaker, until they eventually fail and cause a premature failure ofthe whole battery. Voltage equalization (sometimes called cellbalancing) is a way of compensating for these weaker cells by equalizingthe charge on all the cells in the battery, thus extending the battery'slife. In addition, this process also serves to prevent the overchargingof individual cells.

Even in a relatively stable battery, small differences in charge, rateof discharge, cell parameters, and cell age can result in different cellvoltages for individual cells after a few charge and discharge cycles,leading to undesirable stress on the battery as the individual cellvoltages begin to slide out of synchronization.

Therefore, in order to obtain good performance and lifespan for mostrechargeable batteries, such as Li-Ion batteries, each battery cellrequires additional circuitry. The level of sophistication of thesecircuits varies widely and so does the cost. The simplest and lowestcost devices typically provide only passive equalization. In such adevice, should the voltage of an individual cell exceed a preset level,a simple discharge circuit attached to the cell is activated to bringthe cell voltage back to what is considered a “normal level.” In such adevice, any “excess” charge is wasted, typically turned into heat.

It would be desirable, therefore to provide a system and method ofactively equalizing charge among cells in a multiple-cell battery. Sucha system could periodically monitor the voltage in each battery cell ina battery or battery portion, and equalize the voltages in the batteriesso that they all remain at the same voltage, or at least close to thesame voltage. Furthermore, it would also be desirable if any chargeremoved from an overcharged battery were not wasted, but was insteadmoved to an undercharged battery instead, avoiding any wasted charge.

However, there is also a competing interest to keep the size, cost, andcomplexity of batteries low. Therefore, it would also be desirable forany active battery equalization system to be simple, energy-efficientsmall, and low cost.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures where like reference numerals refer toidentical or functionally similar elements and which together with thedetailed description below are incorporated in and form part of thespecification, serve to further illustrate an exemplary embodiment andto explain various principles and advantages in accordance with thepresent invention.

FIG. 1 is a circuit diagram of a multiple-cell battery and a batteryequalization circuit according to a disclosed embodiment;

FIG. 2 is a graph of a square wave output by the AC voltage source inFIG. 1, along with an indication as to how it is used to charge anddischarge individual battery cells, according to a disclosed embodiment;

FIG. 3 is a circuit diagram of a cluster for use in a multiple-cellbattery equalization circuit according to a disclosed embodiment;

FIG. 4 is a circuit diagram of voltage equalization circuit including asingle isolated ADC that serves all battery cells within a cluster, andmultiplexing in the analog domain according to a disclosed embodiment;

FIG. 5 is a circuit diagram of a multiple-cell battery and a batteryequalization circuit having a plurality of clusters according to adisclosed embodiment;

FIG. 6 is flow chart of a battery equalization operation according to adisclosed embodiment.

DETAILED DESCRIPTION

The instant disclosure is provided to further explain in an enablingfashion the best modes of performing one or more embodiments of thepresent invention. The disclosure is further offered to enhance anunderstanding and appreciation for the inventive principles andadvantages thereof, rather than to limit in any manner the invention.The invention is defined solely by the appended claims including anyamendments made during the pendency of this application and allequivalents of those claims as issued.

It is further understood that the use of relational terms such as firstand second, and the like, if any, are used solely to distinguish onefrom another entity, item, or action without necessarily requiring orimplying any actual such relationship or order between such entities,items or actions. It is noted that some embodiments may include aplurality of processes or steps, which can be performed in any order,unless expressly and necessarily limited to a particular order; i.e.,processes or steps that are not so limited may be performed in anyorder.

Much of the inventive functionality and many of the inventive principleswhen implemented, may be supported with or in integrated circuits (ICs),such as dynamic random access memory (DRAM) devices, static randomaccess memory (SRAM) devices, or the like. In particular, they may beimplemented using CMOS transistors. It is expected that one of ordinaryskill, notwithstanding possibly significant effort and many designchoices motivated by, for example, available time, current technology,and economic considerations, when guided by the concepts and principlesdisclosed herein will be readily capable of generating such ICs withminimal experimentation. Therefore, in the interest of brevity andminimization of any risk of obscuring the principles and conceptsaccording to the present invention, further discussion of such ICs willbe limited to the essentials with respect to the principles and conceptsused by the exemplary embodiments.

Battery Equalization Circuit

FIG. 1 is a circuit diagram of a multiple-cell battery and a batteryequalization circuit 100 according to a disclosed embodiment. As shownin FIG. 1, the multiple-cell battery and a battery equalization circuit100 is formed on a first side 110 and a second side 115 of a transformercore 120. A cell equalization circuit 130 is formed on the first side110 of the transformer core 120 and is connected to a multiple-cellbattery 140. An AC voltage source 135 is provided on the second side ofthe transformer core 120.

The cell equalization circuit 130 includes first through third voltagemeasuring circuits (VMCs) 151, 153, 155, first through thirdanalog-to-digital converters (ADCs) 161, 163, 165, first through fourthtriacs 171, 173, 175, 177, and first through third secondary transformerwindings 181, 183, 185. The multiple-cell battery 140 includes firstthrough third battery cells 141, 143, 145 connected in series with eachother. The AC voltage source 135 includes a primary transformer winding190 and an AC bi-directional power signal generator 195.

The first through third battery cells 141, 143, 145 are connected inseries with each other to provide a total voltage of the multiple-cellbattery 140. This total voltage is simply the sum of the individual cellvoltages of the first through third battery cells 141, 143, 145.

The primary transformer winding 190 is connected to a first side of thetransformer core 120, and is attached to the output of the ACbi-directional power signal generator 195. The AC signal generator 195operates to generate an AC signal that is provided to the cellequalization circuit 130 through the transformer 120 via the primarytransformer winding 190. In the disclosed embodiment, the AC signal is asquare wave signal.

Although the embodiment disclosed with respect to FIG. 1 discloses theuse of a square wave, other forms of bipolar power signals can be usedin alternate embodiments for example a semi-square wave or any suitablebipolar power signal can be provided by the AC bi-directional powersignal generator 195.

In the disclosed embodiment, the AC signal generator 195 is powered bythe multiple-cell battery 140. The AC bi-directional power signalgenerator 195 receives the full voltage of the entire battery andconverts it onto the AC signal (i.e., the square wave). Furthermore, bypowering the AC signal generator 195 from the battery, the amplitude ofthe AC signal is proportional to the voltage of the battery. This makesit easier for an equalization control circuit to determine the optimalbattery cell voltage.

The first through third voltage measuring circuits 151, 153, 155, areeach connected between the positive and negative terminals of arespective one of the first through third battery cells 141, 143, 145.The first through third voltage measuring circuits 151, 153, 155 operateto respectively determine first through third analog voltages of each ofthe first through third battery cells 141, 143, 145.

The first through third ADCs 161, 163, 165, receive the first throughthird analog voltages from the first through third voltage measuringcircuits 151, 153, 155, respectively, and to convert them into firstthrough third digital voltages. These digital voltages are then providedto a control circuit (not shown).

The first through third secondary transformer windings 181, 183, 185 areconnected in series with each other, and are each individually connectedto a second side of the transformer core 120. Thus, each of the firstthrough third secondary transformer windings 181, 183, 185 is connectedto the primary winding 190 through the transformer core 120, and canreceive the AC signal generated by the AC signal generator 195.Furthermore, the use of a transformer to pass this AC signal allows thecell equalization circuit 130 to be electrically isolated from the ACsignal generator 195.

The first triac 171 is connected at one end to a first (top) node of thefirst secondary transformer winding 181 and at the other end to apositive terminal of the first battery cell 141. The second and thirdtriacs 173, 175 are each connected at one end to a node between adjacentsecondary transformer windings 181, 183, 185, and at the other end to anode between adjacent battery cells (i.e., where a positive terminal ofone battery is connected to a negative terminal of another battery). Thefourth triac 177 is connected at one end to a second (bottom) node ofthe third secondary transformer winding 183 and at the other end to anegative terminal of the third battery cell 145.

When activated, a triacs becomes conductive, allowing current to flowthrough it in either direction. Thus, in the cell equalization circuit130, the triacs operate as bi-directional switches between the batterycells 141, 143, 145 and the secondary transformer windings 181, 183,185. In this embodiment, the triacs 171, 173, 175, 177 are eachoptically-coupled triacs that are controlled by the operation of alight-emitting diode in the triac. Because of this, the triacs 171, 173,175, 177 are electrically isolated from the control circuit (not shown)that controls the operation of the light-emitting diodes in theoptically-coupled triacs 171, 173, 175, 177.

When adjacent pairs of triacs (e.g., the first and second triacs 171,173; the second and third triacs, 173, 175; and the third and fourthtriacs 175, 177) are activated, a respective battery cell 141, 143, 145is connected to its respective secondary transformer winding 181, 183,185, allowing it to receive the AC voltage generated by the AC signalgenerator 195. This allows the selected battery cell to either becharged or discharged based on the polarity of the AC signal generatedby the AC signal generator 195. Furthermore, there is no need for anyadditional control signals to turn the triacs 171, 173, 175, 177 off,since the triacs 171, 173, 175, 177 automatically shut down when theyencounter a zero crossing. At such a zero crossing, the current througha given triac will be zero, which will be below its holding current,causing it to become non-conductive.

Although the system of FIG. 1 is shown with a multiple-cell battery 140that has three battery cells 141, 143, 145, in alternate embodiments themultiple-cell battery 140 could have more or fewer cells. In such cases,the cell equalization circuit 130 would be modified accordingly toinclude a summer, an ADC, a triac, and a secondary transformer windingfor each cell in the multiple-cell battery.

A battery equalization circuit is provided, comprising: a first positivebattery node configured to connect to a positive node of a firstmonitored battery cell contained in a battery circuit that includes aplurality of other battery cells connected in series with the firstmonitored battery cell; a first negative battery node configured toconnect to a negative node of the first monitored battery cell; a firsttransformer winding configured to receive an output voltage of an ACgenerator, the first transformer winding having a first uppertransformer node and a first lower transformer node; a first upper triacconnected between the first positive battery node and the first uppertransformer node; a first lower triac connected between the firstnegative battery node and the first lower transformer node; and amultiplexers configured to control the operation of the first uppertriac and the first lower triac based on instructions received from acentral control unit, wherein the instructions received from the centralcontrol unit are generated based on a first measured cell voltagebetween the first positive battery node and the first negative batterynode, and a total battery voltage of the battery circuit.

The first upper triac and the first lower triac may both beoptically-coupled triacs.

The first monitored battery cell and the plurality of other batterycells may all be configured to have the same preset voltage, and thetotal battery voltage of the battery circuit may be determined bymeasuring a voltage of all of the first monitored battery cell and theplurality of other battery cells arranged in series with each other, anddividing the resulting voltage by a total number of battery cells,including the first monitored battery cell and the plurality of otherbattery cells.

The battery equalization circuit may further comprise: a firstanalog-to-digital converter connected between the first positive batterynode and the first negative battery node, and configured to determine afirst analog cell voltage, and to convert the first analog cell voltageinto a first digital cell voltage, wherein the central control circuituses the first digital cell voltage as the first measured cell voltage.

The battery equalization circuit may further comprise: a first switchconnected between the first positive node and a voltage-measurementnode; a cluster analog-to-digital converter connected thevoltage-measurement node, and configured to determine a first analogcell voltage at the first positive node when the first switch is closed,and to convert the first analog cell voltage into a first digital cellvoltage; and a cluster isolation element for electrically isolating thecluster analog-to-digital converter from the multiplexer, wherein

the multiplexer controls the operation of the first switch.

The battery equalization circuit may further comprise: a second positivebattery node configured to connect to a positive node of a secondmonitored battery cell selected from the plurality of other batterycells connected in series with the first monitored battery cell; asecond negative battery node configured to connect to a negative node ofthe second monitored battery cell; a second transformer windingconfigured to receive the output voltage of the AC generator, the secondtransformer winding having a second upper transformer node and a secondlower transformer node; and a triac connected between the secondpositive battery node and the second upper transformer node, wherein themultiplexer is further configured to control the operation of the secondtriac based on further instructions received from a central controlunit, wherein the further instructions received from the central controlunit are generated based on a second measured cell voltage between thesecond positive battery node and the second negative battery node, andthe total battery voltage of the battery circuit. The second triac maybe an optically-coupled triac.

The battery equalization circuit may further comprise: a secondanalog-to-digital converter connected between the second positivebattery node and the second negative battery node, and configured todetermine a second analog cell voltage, and to convert the second analogcell voltage into a second digital cell voltage, wherein the centralcontrol circuit uses the second digital cell voltage as the secondmeasured cell voltage.

The battery equalization circuit may further comprise: a second switchconnected between the second positive node and the voltage-measurementnode; wherein the cluster analog-to-digital converter is furtherconfigured to determine a second analog cell voltage at the secondpositive node when the second switch is closed, and to convert thesecond analog cell voltage into a second digital cell voltage, and themultiplexer controls the operation of the second switch.

Voltage Equalization Graph

FIG. 2 is a graph of a square wave output by the AC voltage source 135in FIG. 1, along with an indication as to how it is used to charge anddischarge individual battery cells, according to a disclosed embodiment.As shown in FIG. 2, the AC bi-directional voltage source 135 generates asquare wave 210, which has a positive half above zero volts, and anegative half below zero volts. In particular, half of the squaresgenerated by the AC bi-directional voltage source 135 have a negativevoltage, while the other half have a positive voltage.

One exemplary cycle of the square wave 210 starts at time T₁ with a zerocrossing from −A volts to +A volts. The square wave 210 is maintained at+A volts until time T₂, when another zero crossing takes the square wave210 from +A volts to −A volts. Then, the square wave 210 is maintainedat −A volts until time T₃, when another zero crossing takes the squarewave 210 from −A volts back to +A volts, at which point a new cyclebegins.

In operation, portions of the square wave with the positive voltage +Aare used to charge battery cells 141, 143, 145 in the multiple-cellbattery 140 when they are undercharged. Similarly, portions of thesquare wave with the negative voltage −A are used to discharge batterycells 141, 143, 145 when they are overcharged. This allows theindividual voltages of the battery cells 141, 143, 145 to be adjusted upand down as needed to allow them to more easily maintain a similarvoltage (although the current between the AC power source 135 and thebattery 140 is limited by some system parameters).

For example, if the first battery cell 141 had a low voltage and neededto be charged, a battery controller would activate the first and secondtriacs 171, 173 to connect the first battery cell 141 to the AC voltagesource 135 (e.g., through a transformer, as shown in FIG. 1) at a timeT_(P). The first battery call 141 would accumulate a first charge amount220 until the zero crossing at T₂, at which point the triacs 171, 173would turn off, isolating the first battery cell 141 from the AC voltagesource 135. In this way, the first battery cell 141 would be charged bythe first charge amount 220 provided by the square wave 210 from timeT_(P) to T₂.

Similarly, if the second battery cell 143 had a high voltage and neededto be discharged, the battery controller would activate the second andthird triacs 173, 175 to connect the second battery cell 143 to the ACvoltage source 135 (e.g., through a transformer, as shown in FIG. 1) ata time T_(N). The second battery call 143 would lose a second chargeamount 230 until the zero crossing at T₃, at which point the triacs 173,175 would turn off, isolating the second battery cell 143 from the ACvoltage source 135. In this way, the second battery cell 143 would bedischarged by the second charge amount 230 (from the negative voltage)drained by the square wave 210 from time T_(N) to T₃. The charge isreturned through the transformer and you bidirectional AC/DC circuitryto the battery 140

Since the times T_(P) and T_(N) can be varied, this system allows adesired amount of charge to be added or subtracted from one of thebattery cells 141, 143, 145. All that is necessary is to set the timeT_(P) to achieve a desired first charge amount 220 to be added to theselected battery cell, or the time T_(N) to achieve a desired secondcharge amount 230 to be subtracted from the selected battery cell.

In some embodiments, the battery cells in a multiple-cell battery willbe further divided into a plurality of multiple-cell clusters. In thisway, individual cell equalization clusters can be provided, each ofwhich operates on the battery cells in a particularly battery cluster.

Voltage Equalization Cluster

Two embodiments for reading and multiplexing the voltage of each batterycell are provided in FIGS. 3 and 4. FIG. 3 is a circuit diagram of avoltage equalization circuit using an isolated ADC for each batterycell, and multiplexing in the digital domain. FIG. 4 is a circuitdiagram of voltage equalization circuit including a single isolated ADCthat serves all battery cells within a cluster, and multiplexing in theanalog domain.

As shown in FIG. 3, the voltage-equalization circuit 300 includes firstthrough M^(th) clusters 310A, 310B, . . . , 310D, a common ADC 350, athe common isolation element 360, a cluster control element 370, and aDC/AC conversion circuit 395. The first through M^(th) clusters 310A,310B, . . . , 310D, are connected to first through M^(th) battery cells320A, 320B, . . . , 320D, respectively, and the first through M^(th)battery cell clusters 320A, 320B, . . . , 320D collectively provide DCpower to the DC/AC conversion element 395. Each of the first throughM^(th) clusters 310A, 310B, . . . , 310D, includes first through N^(th)cluster cells 331, 333, . . . , 339, and a multiplexer 375 for receivinga plurality of digital voltage values from the cluster, and providing aplurality of control signals. The multiple-cell battery cluster 320includes first through N^(th) battery cells 321, 329 arranged in serieswith each other. Each of the first through N^(th) cluster cells 331, 339further includes a cell ADC 161, 163, . . . , 169, a cell isolationelement 351, 353, . . . 359, and a triac 171, 173, . . . , 179.

The first multiple-cell battery cluster 320A includes first throughN^(th) battery cells 321A, 323A, 329A arranged in series with eachother. In this way, each of the first through N^(th) battery cells 321A,323A, 329A combine to provide a total voltage for the battery cluster320A. In addition, although the second battery cluster 320B and theM^(th) battery cluster 320D are shown as single battery elements, thisis representational only. Each of the second through M^(th) batteryclusters 320B, . . . , 320D are configured the same as the first batterycluster 320A.

The first through N^(th) cluster cells 331, 333, . . . , 339 areconnected to the first through N^(th) battery cells 321A, 323A, 329A,respectively, in the first battery cluster 320A. Each cluster cell 331,333, . . . , 339 has a connections line, which is connected to thepositive and terminal (node) of a corresponding battery cell.

In each of the first through N^(th) cluster cells 331, 333, . . . , 339,the secondary transformer winding 385 is connected to the connectionline. It is wrapped around a transformer core (not shown) so that it canreceive an AC signal from an AC signal generator (not shown) through thetransformer core.

The triac 171, 173, . . . , 179 in each cluster cell is provided in theconnection lines, between the secondary transformer winding 385 and therespective battery cell. Each triac 171, 173, . . . , 179 operates basedon a control signal provided by the cluster control circuit 360, andprovided through the control bus and the multiplexer 375.

When activated, the triac 171, 173, . . . , 179 becomes conductive,allowing current to flow through it in either direction. Thus, thetriacs 171, 173, . . . , 179 operate as bi-directional switches betweena secondary transformer winding 385 in the cluster cell and a respectivebattery cell. In this embodiment, the triacs 171, 173, . . . , 179 areeach optically-coupled triacs that are controlled by the operation of alight-emitting diode in the triac. Because of this, the triacs 171, 173,. . . , 179 are electrically isolated from the cluster control circuit360 and multiplexer 375 that control the operation of the light-emittingdiodes in the optically-coupled triacs 171, 173, . . . , 179.

The cell ADC 161, 163, . . . , 169 measures an analog voltage for acorresponding battery cell, and converts it to a measured digitalvoltage, which it provides to the cluster control element 370 via themultiplexer 375 and the control bus.

The cell isolation elements 351, 353, . . . , 359 are located betweencorresponding cell ADCs 161, 163, . . . , 169 and the multiplexer 375,and serve to electrically isolate the cell ADCs 161, 163, . . . , 169from the multiplexer 375 and subsequent circuitry.

The common ADC 350 is connected to a positive terminal of the entirebattery, and operates to measure an analog voltage for the entirebattery. In addition, the common ADC 350 converts the measured analogbattery voltage to a measured digital battery voltage, which it providesto the cluster control element 370.

The common isolation element 360 is located between the common ADC 350and the cluster control element 370. The common isolation element 360serves to electrically isolate the common ADC 350 from the clustercontrol element 370.

The cluster control element 370 provides instructions to the multiplexer375 via the control bus, telling the multiplexer 375 which cell ADCoutput to provide the cluster control element 370, and what controlsignals to provide to the triacs 171, 173, . . . , 179. In this way, thecluster control element 370 can control multiplexer 375 to sequentiallyprovide measured digital cell voltages of each of the first throughN^(th) battery cells 321A, 323A, . . . , 329A in the battery cluster320A. It can also control the operation of the triacs 171, 173, . . . ,179 in the first through N^(th) cluster cells 331, 339 to selectivelyconnect the first through N^(th) battery cells 321, 329 to an AC signalprovided to a respective secondary transformer winding 385 so that theycan receive an AC signal.

As shown in FIG. 4, the voltage-equalization circuit 400 includes firstthrough M^(th) clusters 310A, 310B, . . . , 310D, a common ADC 350, athe common isolation element 360, a cluster control element 370, and aDC/AC conversion circuit 395. The first through M^(th) clusters 310A,310B, . . . , 310D, are connected to first through M^(th) battery cells320A, 320B, . . . , 320D, respectively, and the first through ML batterycell clusters 320A, 320B, . . . , 320D collectively provide DC power tothe DC/AC conversion element 395. Each of the first through M^(th)clusters 310A, 310B, . . . , 310D, includes first through N^(th) clustercells 331, 333, . . . , 339, a cluster ADC 455, a cluster isolationelement 460, and a multiplexer 475 for providing a plurality of controlsignals to the cluster cells 441, 443, . . . , 449. The multiple-cellbattery cluster 320 includes first through N^(th) battery cells 321, 329arranged in series with each other. Each of the first through N^(th)cluster cells 331, 339 further includes a cell switch 441, 443, . . . ,449, and a triac 171, 173, . . . , 179.

The voltage-equalization circuit 400 of FIG. 4 operates essentially asthe voltage-equalization circuit 300 of FIG. 3. As a result, only thedifferences between these two circuits will be described.

The voltage-equalization circuit 400 differs from thevoltage-equalization circuit 300 in that individual cells do not includea cell ADC or a cell isolation element. Instead, each individual cellincludes a cell switch 441, 443, . . . , 439, while the cluster as awhole includes a cluster ADC 455 and a cluster isolation element 460.

As a result, the multiplexer 475 does not have to choose betweenmultiple voltage readings from multiple cell ADCs. Rather, the cellswitches 441, 443, . . . , 439 need to be activated to connect one ofthe battery cells to the cluster ADC 455, so that the ADC 455 can readthe analog voltage of that battery cell and convert it to a digitalvoltage.

The cluster isolation element 460 is located between the roster ADC 455and the multiplexer 475. The cluster isolation element 460 serves toelectrically isolate the cluster ADC 455 from the multiplexer 475.

The multiplexer 475 controls the operation of the triacs 171, 173, . . ., 179 as in the embodiment of FIG. 3. In addition, although not shown,the multiplexer can also provide control signals to the cell switches441, 443, . . . , 439, identifying which should be connected to thecluster ADC 455 at what time.

In alternate embodiments, the connection lines in some of the cells in acluster can be arranged such that some of the battery cells in a batterycluster have their positive terminal connected to an upper winding of acorresponding secondary transformer winding and their negative terminalconnected to a lower winding of the corresponding secondary transformerwinding, while the connection lines in other of the cells in the clustercan be arranged such that the other battery cells in the battery clusterhave their negative terminal connected to an upper winding of acorresponding secondary transformer winding and their positive terminalconnected to a lower winding of the corresponding secondary transformerwinding. In one particular embodiment, half of the cells are arranged inone of these formats and the other half of cells arranged in the otherformat.

A battery equalization circuit is provided, comprising: a primarybattery equalization cluster comprising N primary cluster cells; acentral control circuit configured to control the operation of theprimary battery equalization cluster, and to provide signals to a databus; and an isolation element connected between the control circuit andthe data bus, the isolation element being configured to provideelectrical isolation between the control circuit and the data bus,wherein an i^(th) primary cluster cell further comprises an i^(th)positive primary battery node configured to connect to a positive nodeof an i^(th) monitored primary battery cell contained in a primarybattery circuit that includes N primary battery cells connected inseries with each other, an i^(th) negative primary battery nodeconfigured to connect to a negative node of the i^(th) monitored primarybattery cell, an i^(th) primary transformer winding configured toreceive an output voltage of an AC generator, the i^(th) primarytransformer winding having an ith upper primary transformer node and ani^(th) lower primary transformer node, an i^(th) upper primary triacconnected between the i^(th) positive primary battery node and thei^(th) upper primary transformer node, and an i^(th) lower primary triacconnected between the i^(th) negative primary battery node and thei^(th) lower primary transformer node, the control circuit is configuredto control the operation of the i^(th) upper primary triac and thei^(th) lower primary triac based on an i^(th) measured primary cellvoltage between the i^(th) positive primary battery node and the i^(th)negative primary battery node, and a total primary battery voltage ofthe primary battery circuit, and i is an integer index that varies from1 to N.

The battery equalization circuit may further comprise: a secondarybattery equalization cluster comprising M secondary cluster cells,wherein a j^(th) secondary cluster cell further comprises a j^(th)positive secondary battery node configured to connect to a positive nodeof a jth monitored secondary battery cell contained in a secondarybattery circuit that includes N secondary battery cells connected inseries with each other, a j^(th) negative secondary battery nodeconfigured to connect to a negative node of the jth monitored secondarybattery cell, a j^(th) secondary transformer winding configured toreceive the output voltage of the AC generator, the j^(th) secondarytransformer winding having a j^(th) upper secondary transformer node andan j^(th) lower secondary transformer node, a j^(th) upper secondarytriac connected between the j^(th) positive secondary battery node andthe j^(th) upper secondary transformer node, and a j^(th) lowersecondary triac connected between the j^(th) negative secondary batterynode and the j^(th) lower secondary transformer node, the controlcircuit is further configured to control the operation of the j^(th)upper secondary triac and the j^(th) lower secondary triac based on aj^(th) measured secondary cell voltage between the j^(th) positivesecondary battery node and the j^(th) negative secondary battery node,and a total secondary battery voltage of the secondary battery circuit,the central control circuit is further configured to provide signals tothe data bus, and j is an integer index that varies from 1 to M.

An i^(th) primary cluster cell may further comprise: an i^(th) primaryvoltage measuring circuit connected between the i^(th) positive primarybattery node and the an i^(th) negative primary battery node, andconfigured to determine an i^(th) primary analog cell voltage; and ani^(th) primary analog-to-digital converter configured to convert thei^(th) primary analog cell voltage into an i^(th) primary digital cellvoltage, wherein the control circuit uses the i^(th) primary digitalcell voltage as the i^(th) measured primary cell voltage.

The battery equalization circuit may further comprise: an i^(th) upperprimary switch connected between the i^(th) positive primary batterynode and the control circuit; and an i^(th) lower primary switchconnected between the i^(th) negative primary battery node and thecontrol circuit; wherein the control circuit controls the operation ofthe ith upper primary switch and the i^(th) lower primary switch; thecontrol circuit further comprises a voltage measuring circuit connectedto the i^(th) upper primary switch and the i^(th) lower primary switch,and is configured to measure an i^(th) primary analog cell voltage ofthe i^(th) monitored primary battery cell when the i^(th) upper primaryswitch and the i^(th) lower primary switch are both closed, and ananalog-to-digital converter configured to convert the i^(th) primaryanalog cell voltage into an i^(th) primary digital cell voltage, and thecontrol circuit uses the i^(th) primary digital cell voltage as thei^(th) measured primary cell voltage.

Battery Equalization Circuit with Multiple Voltage-Equalization Clusters

FIG. 5 is a circuit diagram of a multiple-cell battery and a batteryequalization circuit 500 having a plurality of clusters 310A, 310B,310C, 410A, 410B, 410C according to a disclosed embodiment. As shown inFIG. 5, the multiple-cell battery and a battery equalization circuit 400includes first through third voltage-equalization clusters 310A, 310B,310C, 410A, 410B, 410C, first through third battery clusters 320A, 320B,320C, an AC signal generator 530, a primary transformer winding 535, atransformer core 540, and a central controller 550.

The first through third voltage-equalization clusters 310A, 310B, 310C,410A, 410B, 410C and the first through third battery clusters 320A,320B, 320C each operate as described above with respect to FIG. 3. Eachvoltage-equalization clusters 310A, 310B, 310C, 410A, 410B, 410C servesto equalize the voltage in the battery cells (e.g., 321A, 329A, 321B,329B, 321C, 329C) contained in the first through third battery clusters320A, 320B, 320C. The first through third battery clusters 320A, 320B,320C are arranged in series to provide a total battery voltage.

An isolated common ADC can be provided to measure the voltage of theentire set of battery clusters (i.e., the entire battery voltage), andprovide that total battery voltage measurement to the central controller550 in a manner similar to that shown in FIGS. 3 and 4.

The AC signal generator 530 operates to provide an AC signal to theprimary transformer winding 535. In this embodiment, the AC signal is asquare wave signal. However, in alternate embodiments this AC signal canbe any appropriate bidirectional signal.

In the disclosed embodiment, the AC signal generator 430 is powered bythe entire voltage of the multiple-cell battery (i.e., a total voltageof the battery clusters 320A, 320B, 320C). In this way, the AC signalgenerator 530 receives the full voltage of the entire battery andconverts it onto the AC signal (i.e., the square wave). Furthermore, bypowering the AC signal generator 530 from the whole battery voltage, theamplitude of the AC signal is proportional to the voltage of the entirebattery. Thus, if the central controller 550 is provided with theamplitude of the AC signal, it can easily pre-determine the totalbattery voltage (although a precise value of the total battery voltageis determined by an isolated common ADC).

The primary transformer winding 535 receives the AC signal generated bythe AC signal generator 530 and provides it to the secondary transformerwindings 385 in each of the cluster cells 331, 339 in each of thevoltage-equalization clusters 310A, 310B, 310C, 410A, 410B, 410C via thetransformer core 540. By providing the AC signal to the cluster cells(e.g., 331, . . . , 339) via a transformer (primary transformer winding435, transformer core 440, secondary transformer winding 385), themultiple-cell battery and a battery equalization circuit 400electrically isolates the voltage-equalization clusters 310A, 310B, 310Cand the battery clusters 320A, 320B, 320C from the AC signal generator430.

The central controller 450 communicates with the first through thirdvoltage-equalization clusters 310A, 310B, 310C, and the AC signalgenerator 430 through a control and data bus. It provides each with allof the control signals they need to operate properly. As noted abovewith respect to FIGS. 3 and 4, the voltage-equalization clusters 310A,310B, 310C, 410A, 410B, 410C each include various isolation elementsthat electrically isolate them from the control and data bus. Since thevoltage-equalization clusters 310A, 310B, 310C, 410A, 410B, 410C areisolated from the AC signal generator 530 by the transformer, 530, 540,385, and are isolated from the central controller 550 by the respectiveisolation elements, the voltage-equalization clusters 310A, 310B, 310C,410A, 410B, 410C are completely isolated.

Method of Battery Equalization

FIG. 6 is flow chart of a battery equalization operation 600 accordingto a disclosed embodiment. This operation can be performed on an entiremultiple-cell battery or on a multiple-cell battery cluster formed in amultiple-cluster battery. A goal of this operation 600 is to equalizethe voltages in each of a plurality of battery cells such that they areall at approximately the same voltage, or that they are at least allapproaching the same voltage.

As shown in FIG. 6, this operation 600 starts (605) and a value of N isset equal to 0. (610) N is an index of the battery cells that are formedtogether in the associated battery or battery cluster and so must beequalized. N_(MAX) represents the total number of battery cells in theassociated battery or battery cluster.

N is then incremented by one (615) to represent moving on to the nextbattery cell to be equalized. On the first pass through this step, N isincremented from 0 to 1, indicating that the first battery cell is to beequalized. Once the last battery cell has been equalized, this step willincrement N from N_(MAX) to (N_(MAX)+1).

The operation 500 then determines whether N is greater than N_(MAX),i.e., whether the last battery cell has been equalized. (620) If N isgreater than N_(MAX) (i.e., every battery cell has been equalized), theprocess will end. (625) If N is not greater than N_(MAX), (i.e., notevery battery cell has been equalized), then the operation 500 proceedsto measure the total battery voltage V_(BAT) of the battery or batterycluster. (630) The total battery voltage V_(BAT) is the sum of thevoltages of the N_(MAX) battery cells that make up the battery.

An optimum cell voltage V_(OPT) is then calculated based on the totalbattery voltage V_(BAT). (635) In the disclosed embodiments, the optimumcell voltage V_(OPT) is equal to the total battery voltage V_(BAT)divided by the total number of battery cells N_(MAX)

$\left( {{i.e.},{V_{OPT} = \frac{V_{BAT}}{N_{MAX}}}} \right).$

Then an individual cell voltage V_(N) is determined. (640) Thisindividual cell voltage V_(N) is the voltage of the N^(th) battery cell(for the current value of N). As noted above in FIGS. 1, 3, and 4, thereare various ways in which this individual cell voltage V_(N) can bemeasured.

The operation 500 then determines whether the individual cell voltageV_(N) is less than V_(OPT) (i.e., whether the N^(th) battery cell isundercharged with respect to the optimum voltage V_(OPT)). (645)

If the individual cell voltage V_(N) is less than V_(OPT) (i.e., theN^(th) battery cell is undercharged), then it will be necessary tocharge the N^(th) battery cell to increase its voltage. This isaccomplished by determining how long during a positive cycle of the ACsignal (the square wave signal in the disclosed embodiment) the N^(th)battery cell should be connected to the AC signal. If this connection isbeing made through the use of triacs (as disclosed in FIGS. 1, 3, and4), then a charging/discharging stopping point will already be known,since the triacs will turn non-conductive at the next zero crossing ofthe AC signal after they are activated. Therefore, with a desired amountof charge and a known ending point, it is only necessary to determine astart point within a positive half-cycle of the AC signal to achieve thedesired amount of charging for the N^(th) battery cell.

Therefore, the operation determines a positive cycle turn-on time T_(P)that will provide the desired amount of charging (650), and turns on theappropriate triacs (i.e., sets them to be conductive) at time T_(P) toconnect the N^(th) battery cell to the positive half-cycle of the ACsignal at time T_(P). (655) As noted above, the N^(th) battery cell willautomatically be disconnected from the AC signal when the positivehalf-cycle ends, the AC signal has a zero crossing, and the triacs willshut off (i.e., become non-conductive).

The operation 500 then determines whether the individual cell voltageV_(N) is greater than V_(OPT) (i.e., whether the N^(th) battery cell isovercharged with respect to the optimum voltage V_(OPT)). (660)

If the individual cell voltage V_(N) is greater than V_(OPT) (i.e., theN^(th) battery cell is overcharged), then it will be necessary todischarge the N^(th) battery cell to decrease its voltage. This isaccomplished by determining how long during a negative cycle of the ACsignal (the square wave signal in the disclosed embodiment) the N^(th)battery cell should be connected to the AC signal. If this connection isbeing made through the use of triacs (as disclosed in FIGS. 1, 3, and4), then a charging/discharging stopping point will already be known,since the triacs will turn non-conductive at the next zero crossing ofthe AC signal after they are activated. Therefore, with a desired amountof discharge and a known ending point, it is only necessary to determinea start point within a negative half-cycle of the AC signal to achievethe desired amount of discharging for the N^(th) battery cell.

Therefore, the operation determines a negative cycle turn-on time T_(N)that will provide the desired amount of discharging (665), and turns onthe appropriate triacs (i.e., sets them to be conductive) at time T_(N)to connect the N^(th) battery cell to the negative half-cycle of the ACsignal at time T_(N). As noted above, the N^(th) battery cell willautomatically be disconnected from the AC signal when the negativehalf-cycle ends, the AC signal has a zero crossing, and the triacs willshut off (i.e., become non-conductive).

If the individual cell voltage V_(N) is neither greater than nor lessthan the optimum voltage V_(OPT), the N^(th) battery cell has theoptimum voltage V_(OPT), and no correction need be made.

Once any necessary voltage optimization is performed (either throughcharging or discharging the N^(th) battery cell), operation returns tostep 615, where N is incremented by 1, moving to the next battery cell,and operation continues.

In this way, every battery cell in a battery or battery cluster can bechecked and equalized. In this embodiment the total battery voltageV_(BAT) is newly measured during the equalization operation of a givenbattery cell. This is because the total voltage of the battery willfluctuate slightly as individual battery cells are charged anddischarged. However, in alternate embodiments the total battery voltageV_(BAT) could be measured once at the beginning of the batteryequalization process and that value for V_(BAT) could be used for thefirst through N_(MAX) ^(th) battery cells.

A method of equalizing a battery is provided, including: measuring atotal voltage of a battery containing N battery cells arranged in serieswith each other; determining an optimum cell voltage based on themeasured total voltage and the value of N; measuring a selected cellvoltage of a selected battery cell from the N battery cells; determiningwhether the measured cell voltage is less than the optimum cell voltage;connecting the selected battery cell to a voltage output of a generatorduring a portion of a positive cycle of the voltage output of thegenerator, when it is determined that the measured cell voltage is lessthan the optimum cell voltage; determining whether the measured cellvoltage is greater than the optimum cell voltage; and connecting theselected battery cell to the voltage output of the generator during anegative cycle of the voltage output of the generator, when it isdetermined that the measured cell voltage is greater than the optimumcell voltage, where N is an integer greater than 1.

The optimum cell voltage may be equal to the measured total voltagedivided by the total number of the battery cells in the plurality ofbattery cells.

The operations of measuring the total voltage of the battery,determining the optimum cell voltage, measuring the selected cellvoltage, determining whether the measured cell voltage is less than theoptimum cell voltage, connecting the selected battery cell to thevoltage output of a generator during a positive cycle of the generatorwhen it is determined that the measured cell voltage is less than theoptimum cell voltage, determining whether the measured cell voltage isgreater than the optimum cell voltage, and connecting the selectedbattery cell to the voltage output of the generator during a negativecycle of the generator when it is determined that the measured cellvoltage is greater than the optimum cell voltage may be repeated N timesto allow each of the N battery cells to serve as the selected cell once.

The operation of connecting the selected battery cell to the voltageoutput of a generator during a positive cycle of the generator when itis determined that the measured cell voltage is less than the optimumcell voltage may further comprise: determining a connection duration ofthe positive cycle during which the selected cell should be connected tothe voltage output of the generator, determining a connection startingpoint during the positive cycle at which the selected cell should beconnected to the voltage output of the generator so that the selectedcell will be connected to the voltage output of the generator for theconnection duration, when a connection ending point during the positivecycle at which the selected cell will be disconnected from the voltageoutput of the generator is a zero crossing of the voltage output of thegenerator, connecting the selected battery cell to the voltage output ofthe generator at the determined connection starting point; anddisconnecting the selected battery cell from the voltage output of thegenerator at a next zero crossing of the voltage output of thegenerator.

The operation of connecting the selected battery cell to the voltageoutput of the generator at the determined connection starting point mayfurther comprise: activating a first triac connected between a positiveterminal of the selected battery cell and a first terminal of the outputvoltage of the generator to connect the positive terminal of selectedbattery cell to the first terminal of the output voltage of thegenerator; and activating a second triac connected between a negativeterminal of the selected battery cell and a second terminal of theoutput voltage of the generator to connect the negative terminal ofselected battery cell to the second terminal of the output voltage ofthe generator.

The selected battery cell may be connected to the output voltage of thegenerator through a transformer.

The operation of connecting the selected battery cell to the voltageoutput of a generator during a negative cycle of the generator when itis determined that the measured cell voltage is greater than the optimumcell voltage may further comprise: determining a connection duration ofthe negative cycle during which the selected cell should be connected tothe voltage output of the generator; determining a connection startingpoint during the negative cycle at which the selected cell should beconnected to the voltage output of the generator so that the selectedcell will be connected to the voltage output of the generator for theconnection duration, when a connection ending point during the positivecycle at which the selected cell will be disconnected from the voltageoutput of the generator is a zero crossing of the voltage output of thegenerator; connecting the selected battery cell to the voltage output ofthe generator at the determined connection starting point; anddisconnecting the selected battery cell from the voltage output of thegenerator at a next zero crossing of the voltage output of thegenerator.

The operation of connecting the selected battery cell to the voltageoutput of the generator at the determined connection starting point mayfurther comprise: activating a first triac connected between a positiveterminal of the selected battery cell and a first terminal of the outputvoltage of the generator to connect the positive terminal of selectedbattery cell to the first terminal of the output voltage of thegenerator, and activating a second triac connected between a negativeterminal of the selected battery cell and a second terminal of theoutput voltage of the generator to connect the negative terminal ofselected battery cell to the second terminal of the output voltage ofthe generator.

CONCLUSION

This disclosure is intended to explain how to fashion and use variousembodiments in accordance with the invention rather than to limit thetrue, intended, and fair scope and spirit thereof. The foregoingdescription is not intended to be exhaustive or to limit the inventionto the precise form disclosed. Modifications or variations are possiblein light of the above teachings. The embodiment(s) was chosen anddescribed to provide the best illustration of the principles of theinvention and its practical application, and to enable one of ordinaryskill in the art to utilize the invention in various embodiments andwith various modifications as are suited to the particular usecontemplated. All such modifications and variations are within the scopeof the invention as determined by the appended claims, as may be amendedduring the pendency of this application for patent, and all equivalentsthereof, when interpreted in accordance with the breadth to which theyare fairly, legally, and equitably entitled. The various circuitsdescribed above can be implemented in discrete circuits or integratedcircuits, as desired by implementation.

What is claimed is:
 1. A method of equalizing a battery, including:measuring a total voltage of a battery containing N battery cellsarranged in series with each other; determining an optimum cell voltagebased on the measured total voltage and the value of N; measuring aselected cell voltage of a selected battery cell from the N batterycells; determining whether the measured cell voltage is less than theoptimum cell voltage; connecting the selected battery cell to a voltageoutput of a generator during a portion of a positive cycle of thevoltage output of the generator, when it is determined that the measuredcell voltage is less than the optimum cell voltage; determining whetherthe measured cell voltage is greater than the optimum cell voltage; andconnecting the selected battery cell to the voltage output of thegenerator during a negative cycle of the voltage output of thegenerator, when it is determined that the measured cell voltage isgreater than the optimum cell voltage, where N is an integer greaterthan 1; wherein the operations of measuring the total voltage of thebattery, determining the optimum cell voltage, measuring the selectedcell voltage, determining whether the measured cell voltage is less thanthe optimum cell voltage, connecting the selected battery cell to thevoltage output of a generator during a positive cycle of the generatorwhen it is determined that the measured cell voltage is less than theoptimum cell voltage, determining whether the measured cell voltage isgreater than the optimum cell voltage, and connecting the selectedbattery cell to the voltage output of the generator during a negativecycle of the generator when it is determined that the measured cellvoltage is greater than the optimum cell voltage are repeated N times toallow each of the N battery cells to serve as the selected cell once. 2.The method of claim 1, wherein the optimum cell voltage is equal to themeasured total voltage divided by the total number of the battery cellsin the plurality of battery cells.
 3. A method of equalizing a battery,including: measuring a total voltage of a battery containing N batterycells arranged in series with each other; determining an optimum cellvoltage based on the measured total voltage and the value of N;measuring a selected cell voltage of a selected battery cell from the Nbattery cells; determining whether the measured cell voltage is lessthan the optimum cell voltage; connecting the selected battery cell to avoltage output of a generator during a portion of a positive cycle ofthe voltage output of the generator, when it is determined that themeasured cell voltage is less than the optimum cell voltage; determiningwhether the measured cell voltage is greater than the optimum cellvoltage; and connecting the selected battery cell to the voltage outputof the generator during a negative cycle of the voltage output of thegenerator, when it is determined that the measured cell voltage isgreater than the optimum cell voltage, where N is an integer greaterthan 1; wherein the operation of connecting the selected battery cell tothe voltage output of a generator during a positive cycle of thegenerator when it is determined that the measured cell voltage is lessthan the optimum cell voltage further comprises: determining aconnection duration of the positive cycle during which the selected cellshould be connected to the voltage output of the generator; determininga connection starting point during the positive cycle at which theselected cell should be connected to the voltage output of the generatorso that the selected cell will be connected to the voltage output of thegenerator for the connection duration, when a connection ending pointduring the positive cycle at which the selected cell will bedisconnected from the voltage output of the generator is a zero crossingof the voltage output of the generator; connecting the selected batterycell to the voltage output of the generator at the determined connectionstarting point; and disconnecting the selected battery cell from thevoltage output of the generator at a next zero crossing of the voltageoutput of the generator.
 4. The method of claim 3, wherein the operationof connecting the selected battery cell to the voltage output of thegenerator at the determined connection starting point further comprises:activating a first triac connected between a positive terminal of theselected battery cell and a first terminal of the output voltage of thegenerator to connect the positive terminal of selected battery cell tothe first terminal of the output voltage of the generator; andactivating a second triac connected between a negative terminal of theselected battery cell and a second terminal of the output voltage of thegenerator to connect the negative terminal of selected battery cell tothe second terminal of the output voltage of the generator.
 5. Themethod of claim 4, wherein the selected battery cell is connected to theoutput voltage of the generator through a transformer.
 6. A method ofequalizing a battery, including: measuring a total voltage of a batterycontaining N battery cells arranged in series with each other;determining an optimum cell voltage based on the measured total voltageand the value of N; measuring a selected cell voltage of a selectedbattery cell from the N battery cells; determining whether the measuredcell voltage is less than the optimum cell voltage; connecting theselected battery cell to a voltage output of a generator during aportion of a positive cycle of the voltage output of the generator, whenit is determined that the measured cell voltage is less than the optimumcell voltage; determining whether the measured cell voltage is greaterthan the optimum cell voltage; and connecting the selected battery cellto the voltage output of the generator during a negative cycle of thevoltage output of the generator, when it is determined that the measuredcell voltage is greater than the optimum cell voltage, where N is aninteger greater than 1; wherein the operation of connecting the selectedbattery cell to the voltage output of a generator during a negativecycle of the generator when it is determined that the measured cellvoltage is greater than the optimum cell voltage further comprises:determining a connection duration of the negative cycle during which theselected cell should be connected to the voltage output of thegenerator; determining a connection starting point during the negativecycle at which the selected cell should be connected to the voltageoutput of the generator so that the selected cell will be connected tothe voltage output of the generator for the connection duration, when aconnection ending point during the positive cycle at which the selectedcell will be disconnected from the voltage output of the generator is azero crossing of the voltage output of the generator; connecting theselected battery cell to the voltage output of the generator at thedetermined connection starting point; and disconnecting the selectedbattery cell from the voltage output of the generator at a next zerocrossing of the voltage output of the generator.
 7. The method of claim6, wherein the operation of connecting the selected battery cell to thevoltage output of the generator at the determined connection startingpoint further comprises: activating a first triac connected between apositive terminal of the selected battery cell and a first terminal ofthe output voltage of the generator to connect the positive terminal ofselected battery cell to the first terminal of the output voltage of thegenerator; and activating a second triac connected between a negativeterminal of the selected battery cell and a second terminal of theoutput voltage of the generator to connect the negative terminal ofselected battery cell to the second terminal of the output voltage ofthe generator.
 8. The method of claim 7, wherein the selected batterycell is connected to the output voltage of the generator through atransformer.
 9. A method of equalizing a plurality of battery cells by abattery equalization circuit, the method comprising: measuring a totalvoltage of the plurality of battery cells containing N battery cells,where N is an integer greater than 1; determining by a control circuitof the battery equalization circuit an optimum cell voltage based on themeasured total voltage and the value of N; measuring a selected cellvoltage of a selected battery cell from the plurality of battery cells;determining by the control circuit whether the measured cell voltage isless than or greater than the optimum cell voltage; if it is determinedthat that the measured cell voltage is less than the optimum cellvoltage, connecting the selected battery cell to a voltage output of agenerator during a portion of a positive cycle of the voltage output ofthe generator; if it is determined that that the measured cell voltageis greater than the optimum cell voltage, connecting the selectedbattery cell to the voltage output of a generator during a portion of anegative cycle of the voltage output of the generator; wherein theoperations of measuring the total voltage of the battery, determiningthe optimum cell voltage, measuring the selected cell voltage,determining whether the measured cell voltage is less than or greaterthan the optimum cell voltage, if it is determined that that themeasured cell voltage is less than the optimum cell voltage, connectingthe selected battery cell to a voltage output of a generator during aportion of a positive cycle of the voltage output of the generator, andif it is determined that that the measured cell voltage is greater thanthe optimum cell voltage, connecting the selected battery cell to thevoltage output of a generator during a portion of a negative cycle ofthe voltage output of the generator are repeated N times to allow eachof the N battery cells of the plurality of battery cells to serve as theselected cell.
 10. The method of claim 9, wherein the optimum cellvoltage is equal to the measured total voltage divided by N.
 11. Amethod of equalizing a plurality of battery cells by a batteryequalization circuit, the method comprising: measuring a total voltageof the plurality of battery cells containing N battery cells, where N isan integer greater than 1; determining by a control circuit of thebattery equalization circuit an optimum cell voltage based on themeasured total voltage and the value of N; measuring a selected cellvoltage of a selected battery cell from the plurality of battery cells;determining by the control circuit whether the measured cell voltage isless than or greater than the optimum cell voltage; if it is determinedthat that the measured cell voltage is less than the optimum cellvoltage, connecting the selected battery cell to a voltage output of agenerator during a portion of a positive cycle of the voltage output ofthe generator; if it is determined that that the measured cell voltageis greater than the optimum cell voltage, connecting the selectedbattery cell to the voltage output of a generator during a portion of anegative cycle of the voltage output of the generator; wherein theoperation of connecting the selected battery cell to a voltage output ofa generator during a portion of a positive cycle of the voltage outputof the generator further comprises: determining by the control circuit,a connection duration of the positive cycle during which the selectedcell should be connected to the voltage output of the generator;determining by the control circuit, a connection starting point duringthe positive cycle at which the selected cell should be connected to thevoltage output of the generator so that the selected cell will beconnected to the voltage output of the generator for the connectionduration, when a connection ending point during the positive cycle atwhich the selected cell will be disconnected from the voltage output ofthe generator is a zero crossing of the voltage output of the generator;connecting the selected battery cell to the voltage output of thegenerator at the determined connection starting point; and disconnectingthe selected battery cell from the voltage output of the generator at anext zero crossing of the voltage output of the generator.
 12. Themethod of claim 11, wherein the operation of connecting the selectedbattery cell to the voltage output of the generator at the determinedconnection starting point further comprises: activating a first triacconnected between a positive terminal of the selected battery cell and afirst terminal of the output voltage of the generator to connect thepositive terminal of selected battery cell to the first terminal of theoutput voltage of the generator; and activating a second triac connectedbetween a negative terminal of the selected battery cell and a secondterminal of the output voltage of the generator to connect the negativeterminal of selected battery cell to the second terminal of the outputvoltage of the generator.
 13. The method of claim 12, wherein theselected battery cell is connected to the output voltage of thegenerator through a transformer.
 14. A method of equalizing a pluralityof battery cells by a battery equalization circuit, the methodcomprising: measuring a total voltage of the plurality of battery cellscontaining N battery cells, where N is an integer greater than 1;determining by a control circuit of the battery equalization circuit anoptimum cell voltage based on the measured total voltage and the valueof N; measuring a selected cell voltage of a selected battery cell fromthe plurality of battery cells; determining by the control circuitwhether the measured cell voltage is less than or greater than theoptimum cell voltage; if it is determined that that the measured cellvoltage is less than the optimum cell voltage, connecting the selectedbattery cell to a voltage output of a generator during a portion of apositive cycle of the voltage output of the generator; if it isdetermined that that the measured cell voltage is greater than theoptimum cell voltage, connecting the selected battery cell to thevoltage output of a generator during a portion of a negative cycle ofthe voltage output of the generator; wherein the operation of connectingthe selected battery cell to the voltage output of a generator during aportion of a negative cycle of the voltage output of the generatorfurther comprises: determining by the control circuit, a connectionduration of the negative cycle during which the selected cell should beconnected to the voltage output of the generator; determining by thecontrol circuit, a connection starting point during the negative cycleat which the selected cell should be connected to the voltage output ofthe generator so that the selected cell will be connected to the voltageoutput of the generator for the connection duration, when a connectionending point during the positive cycle at which the selected cell willbe disconnected from the voltage output of the generator is a zerocrossing of the voltage output of the generator; connecting the selectedbattery cell to the voltage output of the generator at the determinedconnection starting point; and disconnecting the selected battery cellfrom the voltage output of the generator at a next zero crossing of thevoltage output of the generator.
 15. The method of claim 14, wherein theoperation of connecting the selected battery cell to the voltage outputof the generator at the determined connection starting point furthercomprises: activating a first triac connected between a positiveterminal of the selected battery cell and a first terminal of the outputvoltage of the generator to connect the positive terminal of selectedbattery cell to the first terminal of the output voltage of thegenerator; and activating a second triac connected between a negativeterminal of the selected battery cell and a second terminal of theoutput voltage of the generator to connect the negative terminal ofselected battery cell to the second terminal of the output voltage ofthe generator.
 16. The method of claim 15, wherein the selected batterycell is connected to the output voltage of the generator through atransformer.
 17. A method of equalizing a plurality of battery cells bya battery equalization circuit, the method comprising: measuring a totalvoltage of the plurality of battery cells containing N battery cells,where N is an integer greater than 1; determining by a control circuitof the battery equalization circuit an optimum cell voltage based on themeasured total voltage and the value of N; measuring a selected cellvoltage of a selected battery cell from the plurality of battery cells;determining by the control circuit whether the measured cell voltage isless than or greater than the optimum cell voltage; if it is determinedthat that the measured cell voltage is less than the optimum cellvoltage, connecting the selected battery cell to a voltage output of agenerator during a portion of a positive cycle of the voltage output ofthe generator; if it is determined that that the measured cell voltageis greater than the optimum cell voltage, connecting the selectedbattery cell to the voltage output of a generator during a portion of anegative cycle of the voltage output of the generator; wherein an amountof time that the selected battery cell is connected to the voltageoutput of the generator during the connecting the selected battery cellto a voltage output of a generator during a portion of a positive cycleof the voltage output of the generator or during the connecting theselected battery cell to the voltage output of a generator during aportion of a negative cycle of the voltage output of the generator isdependent upon the difference between the measured cell voltage and theoptimum cell voltage.
 18. A method of equalizing a battery, including:measuring a total voltage of a battery containing N battery cellsarranged in series with each other; determining an optimum cell voltagebased on the measured total voltage and the value of N; measuring aselected cell voltage of a selected battery cell from the N batterycells; determining whether the measured cell voltage is less than theoptimum cell voltage; connecting the selected battery cell to a voltageoutput of a generator during a portion of a positive cycle of thevoltage output of the generator, when it is determined that the measuredcell voltage is less than the optimum cell voltage; determining whetherthe measured cell voltage is greater than the optimum cell voltage; andconnecting the selected battery cell to the voltage output of thegenerator during a negative cycle of the voltage output of thegenerator, when it is determined that the measured cell voltage isgreater than the optimum cell voltage, where N is an integer greaterthan 1; wherein the an amount of time that the selected battery cell isconnected to the voltage output of the generator during the connectingthe selected battery cell to a voltage output of a generator during aportion of a positive cycle of the voltage output of the generator orduring the connecting the selected battery cell to the voltage output ofthe generator during a negative cycle of the voltage output of thegenerator is dependent upon the difference between the measured cellvoltage and the optimum cell voltage.
 19. The method of claim 17,wherein the operations of measuring the total voltage of the battery,determining the optimum cell voltage, measuring the selected cellvoltage, determining whether the measured cell voltage is less than orgreater than the optimum cell voltage, if it is determined that that themeasured cell voltage is less than the optimum cell voltage, connectingthe selected battery cell to a voltage output of a generator during aportion of a positive cycle of the voltage output of the generator, andif it is determined that that the measured cell voltage is greater thanthe optimum cell voltage, connecting the selected battery cell to thevoltage output of a generator during a portion of a negative cycle ofthe voltage output of the generator are repeated N times to allow eachof the N battery cells of the plurality of battery cells to serve as theselected cell.
 20. The method of claim 18, wherein the operations ofmeasuring the total voltage of the battery, determining the optimum cellvoltage, measuring the selected cell voltage, determining whether themeasured cell voltage is less than the optimum cell voltage, connectingthe selected battery cell to the voltage output of a generator during apositive cycle of the generator when it is determined that the measuredcell voltage is less than the optimum cell voltage, determining whetherthe measured cell voltage is greater than the optimum cell voltage, andconnecting the selected battery cell to the voltage output of thegenerator during a negative cycle of the generator when it is determinedthat the measured cell voltage is greater than the optimum cell voltageare repeated N times to allow each of the N battery cells to serve asthe selected cell once.